Such a method is known in conjunction with different optical systems and is described, for example, in European Patent No. 420 897 B1. In order to generate the heterodyne frequency, which is well-suited for the quantitative evaluation, the radiation emitted by the semiconductor laser (laser diode) is modulated using a time-variable injection current. The radiation emitted by the laser is divided into two beams, one of which is routed via an optical bypass and the other is routed without an optical bypass to the part to be tested. The optical bypass is implemented, for example, using deflecting mirrors or a light guide loop. Details of heterodyne interferometric measurement can be found in the aforementioned document.
Other embodiments for heterodyne interferometric measurement of positions, position changes, rotation angles, speeds and other physical quantities derived therefrom are known, with the optical bypass being implementable using a bypass prism. With these methods and measuring devices, positions or path differences, as well as quantities derived therefrom, can be measured with high accuracy, for example, in the nm range. To modulate the laser, the signal of the injection current in these known methods and devices has a sinusoidal, triangular or sawtooth shape with a rising edge that is flat compared with the pulse period, since only such signal shapes are considered suitable for obtaining reliable measurement results. In these methods, the typical length of the optical bypass, for example, for a heterodyne frequency in the MHZ range as customarily used, is on the order of a few decimeters, for example, 40 cm. This relatively long optical bypass runs counter to the desired miniaturization of measurement systems. In addition, by increasing the length of the optical bypass, the contrast of the interference pattern to be evaluated, which is required for the evaluation and should be as high as possible, is diminished, as can be seen from the coherence function, which shows the drop in contrast with increasing length of the optical bypass AL in the form of an exponential function. In this case, evaluation is made difficult by back reflexes of the optical system onto the laser diode, which results in the otherwise single-mode operation of the laser becoming multimode with a peak-shaped coherence function being obtained and the exponential function being the envelope and dropping more steeply than in single-mode operation. In order to avoid back reflexes, an isolator, for example, must be located upstream from the laser, which results in further expenses.
In another measurement method, i.e, a spectroscopic measurement of exhaled air, it is known from Lachish et al., "Tunable diode laser based spectroscopic system for ammonia detection in human respiration," the Review of Scientific Instrument, Vol. 58, No. 6, June 1987, pp. 923-927, that a semiconductor laser can be controlled using a rectangular modulation current to detect a null signal during consecutive light pulses. The semiconductor laser is temperature stabilized in this method.
U.S. Pat. No. 4,765,738 proposes that, in order to measure the frequency response of an optical receiver system, a heterodyne frequency be generated using a laser diode and an optical bypass, with a rectangular modulation being performed, among other things, in order to control a semiconductor laser. Frequencies up to 10 GHz are to be measured with this method. An optical fiber length of 20 km is proposed.